1. Field of the Invention
The present invention relates to an active matrix type organic light emitting diode (AMOLED) device and, more particularly, to an AMOLED device using a polysilicon thin film transistor (TFT) capable of removing a moire phenomenon and its fabrication method.
2. Description of the Related Art
Recently, a liquid crystal display (LCD) device is widely preferred to be used as a flat panel display due to its light weight and low power consumption. However, since the LCD device is a passive device, not a self-light emitting device, and because of its technical limitations with respect to brightness, contrast, a viewing angle and enlargement of screen, there have been efforts to develop a new flat panel display that can overcome such shortcomings of the LCD device.
An OLED device, one of new flat panel displays, is a self-light emitting type display device having an excellent viewing angle and contrast compared with the conventional LCD device, and because it does not need a backlight, the OLED device is advantageous in terms of power consumption.
In addition, the OLED device also has such advantages that it can be driven at a low DC voltage, has a fast response speed, is resistant to an external impact as being entirely solid, has a wide range of usage temperature, and is low in its fabrication cost.
Especially, as for a fabrication process of the OLED device, unlike the LCD device or a PDP (Plasma Display Panel), the OLED device needs only a deposition and encapsulation equipment, thereby considerably simplifying its fabrication process.
When the OLED device having TFTs is driven by an active matrix method, at each pixel, the same luminance can be obtained by applying a relatively low current, so the OLED device has low power consumption and high resolution and its product can have a large size.
The basic structure and operational characteristics of the active matrix type organic LED (AMOLED) device according to a related art will now be described with reference to FIG. 1.
With reference to FIG. 1, a scan line (gate line) (G) is formed spaced apart in a first direction, and a signal line (data line) (D) and a power supply line (P) are formed in a second direction and crossing the scan line, thereby forming a pixel region.
A switching TFT (TR1), an addressing element, is connected at a crossing of the scan line (G) and the signal line (D), and a storage capacitor (Cst) is connected with the switching TFT (TRI) and the power supply line (P). A driving TFT (TR2), a current source element, is connected with the storage capacitor (Cst) and the power supply line (P), and an EL (electroluminescent) is connected with the driving TFT (TR2).
Herein, the switching TFT (TR1) includes a gate electrode (G1) connected with the scan line (G), a source electrode (S1) connected to the signal line (D) and supplying a data signal, and a drain electrode (D1) connected with a gate electrode (G2) of the driving TFT (TR2), and switches the EL.
The driving TFT (TR2) includes the gate electrode (G2) connected with the drain electrode (D1) of the switching TFT (TR1), a drain electrode (D2) connected with an anode electrode of the EL, and a source electrode (S2) connected with the power line (P) and a ground line.
One electrode of the storage capacitor (Cst) is commonly connected with the drain electrode (D1) of the switching TFT (TR1) and the gate electrode (G2) of the driving TFT (TR2), and the other electrode of the storage capacitor (Cst) is connected with the source electrode (S2) of the driving TFT (TR2) and the power line (P).
The EL includes the anode electrode connected with the drain electrode (D2) of the driving TFT (TR2), a cathode electrode connected with a ground line, and an EL layer formed between the cathode electrode and the anode electrode. The EL layer includes a hole transfer layer, a light emitting layer and an electron transfer layer.
In the above constructed AMOLED device, a current is supplied to the EL through the driving TFT. In this respect, because an existing amorphous silicon TFT has a low carrier mobility, recently, a polysilicon TFT having a carrier mobility better than the amorphous silicon TFT is preferred to be used.
Also, in order to properly exhibit a fine color change, a precise gray scale capability is essential for a display. In the above-mentioned OLED device, an amount of current passing through the EL is controlled. In an active driving method, an amount of current passing through the driving TFT which supplies a current to the EL is controlled to differ an amount of light emission of the OLED device to thereby display the gray scale.
A method for fabricating the general AMOLED as described above will be explained with reference to FIG. 2 according to a related art.
With reference to FIG. 2, a buffer layer 13 is formed on an insulation substrate 11 by using an insulation material such as an oxide film, and a semiconductor layer 15 made of a polysilicon film (poly-Si) is formed on the buffer layer 13 by using a first mask (not shown).
Next, a gate insulation film 17 is formed on the entire surface of the insulation substrate 11 including the semiconductor layer 15, on which a gate electrode material is deposited and then patterned by using a second mask (not shown) to form a gate electrode 19, a lower electrode 21 and a gate line (not shown) simultaneously together.
Subsequently, source electrode region/drain electrode region 15a and 15b are formed by injecting an impurity with certain conductivity, for example, a p-type impurity ion, into the semiconductor layer 15 by using the gate electrode 19 as a mask.
And then, a first interlayer insulation film 23 is deposited on the gate insulation film 17 including the gate electrode 19, on which a conductive material such as a metal is deposited, which is then selectively patterned by using a third mask (not shown) to form a cathode electrode (namely, a pixel electrode) 25 of the EL and an upper electrode 27 of the capacitor at an upper portion of the lower electrode 21 of the storage capacitor Cst.
Subsequently, a second interlayer insulation film 29 is formed on the entire surface including the cathode electrode 25 and the upper electrode 27 of the capacitor, and the second interlayer insulation film 29 and the first interlayer insulation film 23 are selectively patterned by using a fourth mask (not shown) to form first to fifth contact holes 31a˜31e exposing the source/drain regions 15a and 15b and the upper and lower electrodes 27 and 21 of the capacitor Cst.
And then, a conductive material such as a metal is deposited entirely on the substrate including the first to fifth contact holes 31a˜31e and then selectively patterned by using a fifth mask (not shown) to form a first conductive layer pattern 33 for connecting the drain electrode region 15a and the EL, a second conductive layer pattern 35 for connecting the source electrode region 15b and the upper electrode 27 of the capacitor (namely, a power line) and a third conductive layer pattern 37 for connecting the lower electrode 21 of the capacitor, the drain electrode of the first TFT (TR1) and the gate electrode of the second TFT (TR2).
Thereafter, a passivation film 39 is deposited to be thick on the entire surface of the substrate by using an insulation material such as an oxide film, and then, the passivation film 39 and the second interlayer insulation film 29 are selectively patterned by using a sixth mask (not shown) to form an opening 41 exposing the cathode electrode 25.
Subsequently, an EL layer 43, an organic light emitting layer, is formed on the cathode electrode 25 below the opening portion 41, and an anode electrode 45 is formed on the EL layer 43, to fabricate the EL. This results in the related art AMOLED device.
However, according to the above-discussed AMOLED device and its fabrication method according to the related art, after the AMOLED is crystallized by an SLS (Sequential Lateral Solidification), the fabrication process of a lower plate is completed, and then, a single color organic light emitting layer is deposited to perform a DC testing. In this case, as shown in FIG. 3, moiré in the pattern of a comb-teeth is generated, which is a problem.
When the amorphous silicon or the above-described existing polysilicon TFT as the driving TFT (TR2) is used, such a moiré phenomenon does not occur. However, when a 2-shot process leading to excellent productivity is used, such moiré phenomenon occurs.
A major cause of the moiré phenomenon is an interference phenomenon of light. Namely, mutual interference is presumed to work between a regular metal pattern of a pixel and another regular optical layer. The optical layer has different optical characteristics from that of an insulation film and the polysilicon film which undergo a high temperature during a crystallization process.
Such moiré phenomenon affects the characteristics of picture quality so that even a gray is changed, and the teeth of a comb in the moiré phenomenon does not disappear but remains continuously.
The moiré phenomenon causing such problems will be described in more detail.
In order to understand the moiré phenomenon, a beat phenomenon of a sound wave must be first understood.
The beat phenomenon refers to a phenomenon that two waves each with a similar frequency affect each other so a width of a frequency is changed at certain periods according to a difference between the two frequencies of the waves. Waves each with a similar frequency are offset or reinforced by the other frequency to form a certain period by which a wave becomes big or annihilated.
The moiré phenomenon corresponds to a visually generated beat phenomenon. When objects have a certain interval therebetween, an interference fringe is generated between the objects. Thus, in a liquid crystal display panel or in the OLED device, a gate line, a data line, a grain boundary, or the like, forms the moiré pattern on a screen because of a diffractive slit light interference. In this case, the moiré pattern can form a rainbow-shaped grating or the comb-teeth grating.
The moiré phenomenon must be solved with respect to an image display device such as the liquid crystal display (LCD) device, and is quite problematic in case of an EL image display device using the EL as a light emitting device.
An image display device using the EL as a light emitting device has a similarity to the LCD device in that it includes a plurality of gate lines arranged in parallel and a plurality of data lines perpendicularly crossing over the gate lines, where a unit pixel is defined by the crossing of the gate lines and the data lines, and a TFT is provided as a switching device at each unit pixel.
Meanwhile, the EL image display device is different from the LCD device in that (1) it uses the EL which emits light by itself as a light source so it does not require a backlight assembly (a light source of the LCD device), (2) it does not require a liquid crystal layer used in the LCD device because it controls the amount of light emission of the EL by controlling a voltage to display color, and (3) it does not require a color filter layer used in the LCD device for displaying natural color because the EL itself has red, green and blue colors.
Both the EL image display device and the LCD device do have the moiré problem. However, the moiré problem is more severe in the EL that does not have a polarization plate as in the LCD device. Therefore, the moiré phenomenon must be solved for the EL image display device.